Previous studies suggest there is diminished transport capacity of high molecular weight (>30 kDa) material through choroid plexus epithelial cells in the progression of AD. Further, it has been observed that Aβ1-42 levels are significantly lower in serum compared to CSF because of deposition in senile plaques (Solfrizzi, V, D'Introno, A, Colacicco, A M, Capurso, C, Todarello, O, et al. Circulating biomarkers of cognitive decline and dementia Clin Chim Acta, 2006; 364:91-112). There is an ongoing need for methods for detecting biomarkers associated with AS and MCI.